Explaining our new pond health graphs

We're publishing some new graphs in our e-newsletter updates that track average dissolved oxygen levels in our oxidation ponds.

It's natural for a pond's dissolved oxygen levels to fluctuate daily, so we're taking a two-week average to show a high-level picture of how the pond levels are gradually changing.

So did we turn ponds 3 and 4 'orange' when the two-week average is below the benchmark? There are a few reasons:

• The readings in the graphs are averaged out across two weeks. If a pond's dissolved oxygen levels have improved over the past few days, those numbers will be brought down to the fortnightly average.
   
• While dissolved oxygen is one of our main indicators of pond health, we also monitor a range of other parameters, including biological oxygen demand (BOD5), suspended solids, pH levels, sludge levels, nitrogen and phosphorous levels, enterococci and faecal coliforms. If these numbers are trending well, we will move the ponds up a level.
   
• Our wastewater treatment plant operations staff regularly check on the ponds and gauge their health by how they look, the odour levels they're emitting, and the presence of birds. These 'sight-and-sniff tests'  provide a big-picture view of a pond's health, rather than a solely relying on data from a very small section of each pond.

How often should we send you this newsletter?

We've been publishing this e-newsletter weekly since April (and fortnightly before that).

Now that the odours are decreasing, the activated sludge system is operational and the trickling filters are empty, new information and developments relating to the wastewater treatment plant recovery will become less frequent.

So, how often would you like to receive this e-newsletter in the future?

There's a link below to a very short, three-question survey that we'd love you to complete, so we can get an idea of how regularly we should continue sending these updates.

Hold off from cleaning your house

With hydrogen sulphide levels dropping and the days warming up, it might be tempting to clean any discolouration off your house that might have been caused by a chemical reaction between lead paint and hydrogen sulphide. 

We ask that you keep waiting. Before we begin encouraging people to clean their homes, we'd like to see at least four weeks of low hydrogen sulphide levels and improved weather to get an idea of how any discolouration fades. 

We're about to start monitoring some 'test houses' and will provide updates on our website to show how the discolouration improves. 

If you absolutely must clean your house now, make sure you're familiar with our list of dos and don'ts that we published last month. 

Do:

• Read the report findings and information on house discolouration. 
• Wash your walls with a commercial chlorine-based solution, or warm, soapy water. This could help remove any organic dust, common moulds and some discolouration. The presence of these common moulds is not linked to the wastewater treatment plant fire.
• Adhere to the manufacturers’ proprietary ratio recommendations if you use a commercial chlorine-based solution.
• Several washes might be required.

Don't:

• Don't use a high-pressure washer to try and clean any staining or discolouration, as this could result in flakes of lead-based paint entering the environment.
• Don't exceed the manufacturer’s proprietary ratio recommendations if you use a commercial chlorine-based solution.

The Ministry of Health has more information about lead-based paint.

Our team is keeping a register of homes that have reported paint discolouration. If your home has discolouration and you haven't let us know, or if you'd like more advice about what to do, click the green button below. 

 Hydrogen sulphide levels remain very low

Every Wednesday we upload the past week's data for the continuous air monitors that have been set up near the Christchurch Wastewater Treatment plant.

The monitors are to detect the levels of hydrogen sulphide in the Shortland Street and Rudds Road areas, as well as at Bromley and South Brighton Schools.

Over the past two weeks we've noticed a significant and sustained drop in hydrogen sulphide levels across all sites, helped by both the improving oxidation pond health, and the stronger spring breezes that have helped dissipate any odours more quickly.

Note the readings online are only up until Friday 23 September – we'll publish graphs showing this week's increased levels of hydrogen sulphide early next week once the data from all of the sites has been collated.

You can read the data for all locations, and an explanation about how to read the information by clicking the link below. You can also find past readings for each site.

Stay updated

We want everyone to be able to stay updated on our response and recovery from the treatment plant fire, and we regularly post new information and reports on ccc.govt.nz/wastewaterfire 

If you know of anyone who would like to receive this e-newsletter they can subscribe online. 

If you have any questions, or would like further information on the project, please call us on 03 941 8999 or 0800 800 169 or email wastewater@ccc.govt.nz  

Thanks in advance for your patience and understanding as we continue to work on minimising the unpleasant smells after the fire. We’ll be in touch again soon.